Alkali metal lactamate catalyst for the polymerization of diisocyanate with tetracarboxylic acid/dianhydride

ABSTRACT

The use of certain catalysts provide for an improved process for the preparation of soluble polyimides, polyamides, and polyamideimides. The catalysts are compounds of formula ##STR1## wherein M represents an alkali metal, and n is an integer from 2 to 5 inclusive. The improved process comprises reacting organic diisocyanates with polycarboxylic compounds consisting of tetracarboxylic acids or the intramolecular dianhydrides thereof, tricarboxylic acids or the monoanhydrides thereof, dicarboxylic acids, and mixtures thereof, in the presence of said catalysts. The polymers are obtained in solution at low reaction temperatures and short reaction times thereby avoiding side-reactions which otherwise would be detrimental to polymer molecular weight and ultimate polymer properties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a novel process and is more particularlyconcerned with improved processes for the preparation of polyimides,polyamides, and copolymeric mixtures thereof.

2. Description of the Prior Art

The reaction of diisocyanates with dicarboxylic acids and dianhydridesin solution to form polyamides and polyimides is well known in thepolymer art; see for example U.S. Pat. No. 3,592,789 wherein there isdisclosed the formation of coating solutions by reacting a diisocyanate,such as 4,4'-methylenebis(phenylisocyanate) with trimellitic anhydridein dimethylformamide at about 150° to 300° F., and conversion to thecured polymer at 200° to 420° C., U.S. Pat. No. 3,541,038 discloses thepolymerization of trimellitic anhydride with diisocyanates at elevatedtemperatures; and U.S. Pat. No. 3,708,458 discloses the formation ofpolyimides requiring long reaction times. U.S. Pat. No. 3,701,756teaches the use of certain hydroxides and ureas as catalysts for thereaction of isocyanates with anhydrides. However, such catalysts areextremely difficult to remove from the products so obtained. It is knownto those skilled in the polymer art that the reaction of diisocyanateswith dicarboxylic acids in solution to form polyamides requires evenhigher temperatures than those called for in the prior art hereinbeforecited; see for example U.S. Pat. No. 3,642,715.

It has been well established that isocyanates react with some dipolaraprotic solvents such as dimethylformamide, dimethylacetamide,N-methylpyrrolidone, and the like, at elevated temperatures. See M. R.Weiner, J. Org. Chem. 25, 2245 (1960) and S. Terney et al., J. Polym.Sci., Part A-1, 8, 683 (1970). For example, heating of phenylisocyanatein dimethylformamide at only 150° C. for 150 minutes gives a 35% yieldof N-phenyl-N',N'-dimethylformamidine and 30% of a cycloaddition adductderived from a further reaction of the formamidine with four moles ofphenylisocyanate. The side reactions arising during polyermizationsinvolving the use of isocyanates in such solvents, have already beenconsidered; see, The Reaction of Isocyanates with Polar Solvents, by H.Ulrich, paper presented at the University of Detriot, 1974 PolymerConference Series. The side reactions easily lead to chain termination(i.e., lowering of polymer molecular weight), or crosslinking andincorporation of units other than amide or imide into the polymer chain,all of which are highly undesirable when high molecular weight, linearpolymers are desired.

I have now found a process for carrying out the polymerization reactionshereinbefore described and known from the prior art, said process beingfree of the difficulties described hereinabove. The novel process of thepresent invention provides for lower polymerization temperatures, andshorter polymerization times, when compared to the prior art. As anadded advantage to flow from the use of lower reaction temperatures,problems arising from possible solvent -- isocyanate interaction havebeen eliminated. Therefore the soluble polymers obtained by the processof the present invention are characterized by having excellent molecularweight.

SUMMARY OF THE INVENTION

This invention comprises a process for preparing an essentially linear,solvent soluble polyimide, polyamide, or polyamideimide by thecondensation of an organic diisocyanate with the appropriatepolycarboxylic acid derivative in said solvent, the improvement whichcomprises carrying out said process in the presence of a catalyticamount of a compound ##STR2## wherein n is an integer from 2 to 5inclusive and M is an alkali metal.

The term "alkali metal" means sodium, potassium, and lithium.

The term "solvent" means a dipolar aprotic solvent.

The term "appropriate polycarboxylic acid derivative" means adifunctional polycarboxylic compound containing two groups available toreact with the diisocyanate regardless of whether they be two carboxylicacid groups, two intramolecular carboxylic anhydride groups (or the freecarboxylic acids thereof), or one free carboxylic acid group with oneintramolecular anhydride group (or the free carboxylic acids thereof).

DETAILED DESCRIPTION OF THE INVENTION

The process of the invention is applicable to the preparation of anypolyimide, polyamide, or polyamideimide which is soluble, at least tothe extent of about 5 percent by weight, in the reaction solvent used inits preparation. Such polyimides, polyamides, and polyamideimides are awell known class in the art, see for example: U.S. Pat. Nos. 3,063,966,3,541,038, 3,592,789, 3,642,715, 3,692,740, 3,696,077, 3,708,458,3,787,367.

The novel feature of the process of the invention lies in the use of theparticular catalyst set forth above. The procedure employed in carryingout the process of the invention is essentially that employed hithertoin the particular condensation with the notable exception that theaforesaid catalyst is always present in the reaction mixture.

The process of the invention is accomplished in the presence of acatalytic amount of at least one compound of formula (I). By catalyticamount is meant an amount less than 1 mole per mole of isocyanateemployed. The amount of compound (I) employed is advantageously fromabout 0.001 mole to about 0.2 mole per mole of isocyanate, andpreferably is from about 0.002 mole to about 0.02 mole per mole ofisocyanate. Compound (I) in excess of the proportions set forth can beemployed, if desired, but will afford no additional advantage.

The catalysts of formula (I) defined hereinbefore are N-alkali metallactamates and are well known to those skilled in the preparation ofpolyamides from lactams (see U.S. Pat. No. 3,549,580). The alkali metallactamates are commercially available (Foote Mineral Co., Exton, PA) orthey can be easily prepared from the desired lactam and an alkali metalhydride in an inert solvent and the resulting salt obtained by removalof said solvent. Typical examples of the catalysts of formula (I) usedin the process of the present invention include: sodium propiolactamate,potassium propiolactamate, lithium propiolactamate, sodium pyrrolidone(sodium butyrolactamate), potassium pyrrolidone (potassiumbutyrolactamate), lithium pyrrolidone (lithium butyrolactamate), sodiumvalerolactamate, potassium valerolactamate, lithium valerolactamate,sodium caprolactamate, potassium caprolactamate, lithium caprolactamate.A preferred group of catalysts of formulae (I) consist of the alkalimetal salts of pyrrolidone. A particularly preferred catalyst of formula(I) is lithium pyrrolidone.

The process of the present invention is accomplished by bringingtogether in solution under anhydrous conditions, a difunctionalpolycarboxylic compound, an organic diisocyanate and a catalytic amountof a compound of formula (I). It will be recognized by those skilled inthe art that reasonable precautions to exclude moisture should beexercised because of the tendency for isocyanates to react with water.Such precautions include the use of dry solvents, dry apparatus, andcarrying out the reaction under an inert atmosphere, i.e., nitrogen. Thereactants and conditions will be defined in detail hereinafter. In apreferred embodiment of the present invention the difunctionalpolycarboxylic compound and catalyst are dissolved in a dipolar aproticsolvent and the diisocyanate added thereto while the solution is beingheated and stirred. The stirring assists in achieving homogeneity andadvantageously aids in the removal of the carbon dioxide formed duringthe polymerization reaction. While the procedure as set forth above is apreferred embodiment, it is to be understood that the process of thepresent invention can also be readily accomplished by the initialadmixture in solvent of all the ingredients which, upon heating, formthe corresponding polymers in solution. In a most preferred embodiment,the diisocyanate is added, as a solution dissolved in a dipolar aproticsolvent, to the heated solution comprising the polycarboxylic compoundand the catalyst of formula (I).

The process of the present invention is advantageously conducted atelevated temperatures from about 40° C. to about 140° C. and preferablyfrom about 60° C. to about 130° C. Higher reaction temperatures can beemployed, however, such higher temperatures offer no advantage andinsofar as solvent -- isocyanate side reaction can occur thereat, theiruse is not particularly recommended.

The progress of the polymerization reaction is easily monitored by anysuitable analytical method known to one skilled in the polymer art. Aparticularly suitable method is infrared analysis. The characteristicabsorptions arising from the isocyanate groups of the organicdiisocyanate (4.4μ), and the functional groups of the polycarboxyliccompounds such as the anhydride group (5.4μ), the carboxylic acid group(5.85μ ), along with the characteristic absorptions of the polymersobtained therefrom such as the imide group (5.60, 5.80 and 7.25μ), andamide group (6.00μ), allow for the facile determination of the progressand completion of the polymerization. The reaction is continued untilthe diisocyanate and difunctional polycarboxylic compound are no longerdetectable by infrared absorption analysis. The process of the presentinvention is advantageously accomplished in a period from about 2 hoursto about 15 hours and preferably from about 4 hours to about 10 hours.Illustrative of the solvents used in the present invention aredimethylsulfoxide, diethylsulfoxide, dimethylformamide,diethylformamide, dimethylacetamide, diethylacetamide, tetramethylurea,hexamethylphosphoramide, N-methylpyrrolidone, tetramethylenesulfone, andmixtures thereof. A particularly preferred group of solvents consists ofdimethylformamide and N-methylpyrrolidone.

It will be appreciated by one skilled in the art that when mixtures ofdifunctional polycarboxylic compounds hereinafter described, are reactedwith a diisocyanate, the product is a random, or block copolymer,depending on the sequence of polycarboxylic compound addition.

The difunctional polycarboxylic compound employed in the process of theinvention contains at least two carboxylic moieties selected from theclass consisting of free carboxy groups, anhydride groups, and mixturesthereof. Said polycarboxylic compounds are inclusive of aromatic,aliphatic, cycloaliphatic or heterocyclic polycarboxylic acids as wellas the intramolecular anhydrides thereof, provided that, in the case ofthose anhydrides which contain a single anhydride group there is alsopresent in the molecule a free carboxy group.

As will be appreciated by one skilled in the art only thosepolycarboxylic acids which contain carboxy groups attached either to twoadjacent carbon atoms or to two carbon atoms which are separated fromeach other by a single carbon or hetero-atom are capable of formingintramolecular acid anhydrides.

Any of the aforesaid polycarboxylic acids or anhydrides can be employedas the difunctional polycarboxylic compounds in the process of theinvention. As will be apparent to the skilled chemist the nature of therecurring units in the resulting polymers will vary according to thestructure of the starting difunctional polycarboxylic compound.

When the polycarboxylic compound is a dicarboxylic acid which isincapable of forming an intramolecular anhydride, the product formed inaccordance with the process of the invention is a polyamide e.g. theproduct from said dicarboxylic acid and a diisocyanate would contain therecurring unit ##STR3## wherein A is the hydrocarbon residue of thedicarboxylic acid starting material and B is the hydrocarbon residue ofthe diisocyanate. On the other hand, when the polycarboxylic compound isan intramolecular anhydride which contains two anhydride moieties orcontains one anhydride moiety and free carboxylic acid groups capable ofintramolecular anhydride formation, the product of reaction inaccordance with the process of the invention is a polyimide e.g. theproduct of reaction of a diisocyanate and a polycarboxylic compoundcontaining two intramolecular anhydride groups would contain therecurring unit ##STR4## wherein A' is the hydrocarbon residue of thedianhydride and B' is the hydrocarbon residue of the diisocyanate.

Similarly where the polycarboxylic compound contains one anhydride groupin addition to a free carboxylic acid group, the polymer resulting fromthe process of the invention will be a hybrid containing both amide andimide linkages.

All of the above types of polymers can be prepared in accordance withthe novel process hereinabove described and all fall within the scope ofthis invention. Thus, by appropriate choice of the polycarboxyliccompound it is possible to prepare any of a wide variety of polymersusing the single step process of the invention.

Illustrative examples of aromatic dicarboxylic acids employed in theprocess of the present invention include, isophthalic acid andterephthalic acid. Illustrative examples of aliphatic dicarboxylic acidsemployed in the present invention are malonic acid, succinic acid,glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid,sebacic acid, 1-11-undecanedioic acid, 1-12-dodecanedioic acid andbrassylic acid. Illustrative examples of cycloaliphatic dicarboxylicacids include, 1,3-cyclopentanedicarboxylic acid, and1,4-cyclohexanedicarboxylic acid. A particularly preferred aromaticdiacid is isophthalic acid and, a particularly preferred aliphaticdiacid is brassylic acid.

Examples of polycarboxylic compounds which can be employed as the freecarboxylic acids or intramolecular anhydrides thereof, are:

trimellitic acid and the anhydride thereof,

pyromellitic acid and the dianhydride thereof,

mellophanic acid and the anhydride thereof,

benzene-1,2,3,4-tetracarboxylic acid and the dianhydride thereof,

benzene-1,2,3-tricarboxylic acid and the anhydride thereof,

diphenyl-3,3',4,4'-tetracarboxylic acid and the dianhydride thereof,

diphenyl-2,2',3,3'-tetracarboxylic acid and the dianhydride thereof,

naphthalene-2,3,6,7-tetracarboxylic acid and the dianhydride thereof,

naphthalene-1,2,4,5-tetracarboxylic acid and the dianhydride thereof,

naphthalene-1,4,5,8-tetracarboxylic acid and the dianhydride thereof,

decahydronaphthalene-1,4,5,8-tetracarboxylic acid and the dianhydridethereof,

4,8-dimethyl-1,2,3,5,6,7-hexahydronaphthalene-1,2,5,6-tetracarboxylicacid and the dianhydride thereof,

2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic acid and the dianhydridethereof,

2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic acid and the dianhydridethereof,

2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic acid and thedianhydride thereof,

phenanthrene-1,3,9,10-tetracarboxylic acid and the dianhydride thereof,

perylene-3,4,9,10-tetracarboxylic acid and the dianhydride thereof,

bis(2,3-dicarboxyphenyl)methane and the dianhydride thereof,

bis(3,4-dicarboxyphenyl)methane and the dianhydride thereof,

1,1-bis(2,3-dicarboxyphenyl)ethane and the dianhydride thereof,

1,1-bis(3,4-dicarboxyphenyl)ethane and the dianhydride thereof,

2,2-bis(2,3-dicarboxyphenyl)propane and the dianhydride thereof,

2,3-bis(3,4-dicarboxyphenyl)propane and the dianhydride thereof,

bis(3,4-dicarboxyphenyl)sulfone and the dianhydride thereof,

bis(3,4-dicarboxyphenyl)ether and the dianhydride thereof,

ethylene tetracarboxylic acid and the dianhydride thereof,

butane-1,2,3,4-tetracarboxylic acid and the dianhydride thereof,

cyclopentane-1,2,3,4-tetracarboxylic acid and the dianhydride thereof,

pyrrolidine-2,3,4,5-tetracarboxylic acid and the dianhydride thereof,

pyrazine-2,3,5,6-tetracarboxylic acid and the dianhydride thereof,

thiophen-2,3,4,5-tetracarboxylic acid and the dianhydride thereof,

and benzophenone-3,3',4,4'-tetracarboxylic acid and the dianhydridethereof.

Other anhydrides which may be employed in the practice of the inventionare; the intermolecular anhydride of trimellitic acid 1,2-anhydride(see, for example U.S. Pat. No. 3,155,687), the bis-anhydrides disclosedin U.S. Pat. No. 3,277,117 [e.g. 4,4'-ethylene glycol bis-anhydrotrimellitate and 4,4'-(2-acetyl-1,3-glycerol) bis-anhydro trimellitate]and the di-adducts of maleic acid or anhydride with styrene.

While any of the polycarboxylic acids and intramolecular anhydridesthereof defined and exemplified above can be employed in the preparationof the polymers of the invention, a preferred group of compounds forthis purpose are intramolecular anhydrides which are derived frompolycarboxylic acids having at least 3 carboxyl groups of which at leasttwo carboxyl groups are attached directly to an aromatic nucleus inortho-position with respect to each other. A preferred group ofpolycarboxylic acid intramolecular anhydrides are those selected fromthe class consisting of anhydrides having the following formulae##STR5## wherein R₁ represents a group selected from the classconsisting of carboxyl and the group ##STR6## wherein the carbon atomsof the latter are each attached to adjacent carbon atoms in an aromaticring, and wherein X is a bridging group selected from the classconsisting of loweralkylene, carbonyl, sulfonyl and oxygen. The term"loweralkylene" means alkylene containing from 1 to 6 carbon atoms,inclusive, such as methylene, ethylene, 1,3-propylene, 1,4-butylene,2,3-butylene, 1,6-hexylene and the like. A particularly preferred groupconsists of, benzophenone-3,3', 4,4'-tetracarboxylic acid dianhydride,trimellitic anhydride, and mixtures thereof containing from about 10 toabout 90 mole percent of benzophenone-3,3',4,4'-tetracarboxylic aciddianhydride and from about 90 to about 10 mole percent of trimelliticanhydride.

It is to be understood that mixtures of the aforesaid intramolecularanhydrides with the dicarboxylic acid compounds hereinbefore set forthare within the scope of the present invention. A particularly preferredmixture consists of about 80 mole percent of trimellitic anhydride and20 mole percent of isophthalic acid.

The diisocyanates which can be employed in the process of the inventioninclude any of the known diisocyanates. Illustrative of thediisocyanates which are employed in the process of the invention are:2,4-toluene diisocyanate, 2,6-toluene diisocyanate,4,4'-methylenebis(phenylisocyanate), dianisidine diisocyanate, tolidinediisocyanate, 4,4'-diphenylether diisocyanate,4,4'-methylenebis(cyclohexylisocyanate), m-xylene diisocyanate,1,5-naphthalene diisocyanate, and the like. A preferred group ofdiisocyanates consists of, 2,4-toluenediisocyanate,2,6-toluenediisocyanate (and mixtures thereof),4,4'-methylenebis(phenylisocyanate) (MDI), and various mixtures of MDIwith the toluenediisocyanates. A preferred mixture consists of fromabout 10 to about 35 mole percent of 4,4'-methylenebis(phenylisocyanate)and from about 65 to about 90 mole percent of a member selected from thegroup consisting of 2,4-toluenediisocyanate, 2,6-toluenediisocyanate,and mixtures thereof. A particularly preferred mixture consists of about20 mole percent of 4,4'-methylenebis(phenylisocyanate) and about 80 molepercent of a member selected from the group consisting of2,4-toluenediisocyanate, 2,6-toluenediisocyanate, and mixtures thereof.

The proportions of diisocyanate to difunctional polycarboxylic compoundemployed in the process of the present invention are from about 1.0 moleto about 1.10 mole per mole of polycarboxylic compound, and preferablyfrom about 1.0 mole to about 1.05 mole.

Upon completion of the polymerization reaction the polymer can be leftin solution to be used thereafter. In an alternative embodiment, thepolymer is isolated in solid form by standard methods known to thoseskilled in the polymer art. In particular, it is precipitated by pouringthe polymer solution into rapidly stirred water, collection of thepowdered polymer, followed by washing with water and/or non-solvents,and finally drying to the finished material. It will be recognized bythose skilled in the art that isolation of the polymer by precipitationin water will automatically remove the trace amount of catalyst to befound therein. In an optional, and preferred step, the basic catalystpresent in the final solution is neutralized by the addition of a minoramount of an acid, preferably a weak acid such as glacial acetic acid.Such a neutralization step obviates any difficulties that may beencountered when the reaction solution of the polymer is to be useddirectly, i.e., in the making of films, fibers, or coatings.

The polymers prepared by the process of the invention can be employed inany of the uses to which high temperature resistant polyimides orpolyamides are currently put in the art, for example, the polymers ofthe invention in solid form can be molded in the form of bushings, sealfaces, electric insulators, compressor vanes and impellers, pistons andpiston rings, gears, thread guides, cams, brake lining, clutch faces,abrasive articles and the like. They can be employed in solution in thepreparation of coating compositions and can thereby be employed in wirecoating and in the casting or spraying of polymer films on a variety ofsubstrates such as metal, ceramic, fabrics, polymerics and the like.

Indeed, as the polymers prepared by the process of the invention formhigh molecular weight polymers soluble in organic solvents theyrepresent a particularly useful advance in the art since they provide ameans of molding or fabricating high temperature resistant polymers,including fibers, without the need to carry out a final chemicalreaction to produce the polymer in situ. The polymers also findparticular utility in the manufacture of articles having reinforcing ormodifying fillers and the like incorporated therein, including themaking of high temperature resistant laminates. Thus, fillers such asfiberglass, carbon fibers, graphite, molybdenum disulfide (to impartlubricity), powdered metals such as aluminum, copper and the like, andabrasive materials (for producing grinding wheels and the like) can beadded to solutions of the soluble copolyimides of the invention andintimately mixed therewith prior to removal of solvent followed by heatpressing or like techniques necessary to achieve production of thedesired article. Other processing advantages which accrue from the hightemperature resistance, solvent solubility and thermoplasticity of thesecopolyimides of the invention will be apparent to one skilled in theart.

The following examples describe the manner and process of making andusing the invention and set forth the best mode contemplated by theinventors of carrying out the invention but are not to be construed aslimiting.

EXAMPLE 1

A dry 500 ml. resin flask equipped with a stirrer, condenser,thermometer, nitrogen inlet tube, and addition funnel was charged with64.4 g. (0.2 mole) of commercial grade (97.44% anhydride)3,3',4,4'-benzophenonetetracarboxylic acid dianhydride (BTDA) and 0.05g. (0.0005 mole) of lithium pyrrolidone(N-lithium butyrolactam)catalyst. The flask contents were dissolved in 234 g. of drydimethylformamide (distilled over calcium hydride). The temperature ofthe contents was raised to 80° C. and, during constant stirring undernitrogen, a solution consisting of 10.0 g. (0.04 mole) of4,4'-methylenebis(phenylisocyanate) (MDI) and 28.0 g. (0.16 mole) of2,4-toluenediisocyanate (TDI) dissolved in 30 g. of drydimethylformamide (DMF) was added dropwise over 6 hours. The reactionwas continued for another 2 hours at 80° C. At the end of this time,infrared analysis indicated the reaction was complete.

The DMF solution, having an inherent viscosity, ηinh (0.25% at 29.4°C.)=0.54, consisting of approximately 25 percent by weight ofcopolyimide was characterized by a structure wherein approximately 80percent of the recurring copolyimide units had the formula ##STR7## andthe remaining 20 percent of the recurring units had the formula ##STR8##Films were easily cast from this solution and when redissolved in DMFdisplayed an inherent viscosity of ηinh(0.5% at 28.8° C.)=0.59.

EXAMPLE 2

Using the procedure and reactants described in Example 1 except that theDMF was replaced by 175 g. of dry distilled N-methylpyrrolidone (NMP),the quantities of reactants were reduced by one half, and 0.02 g.(0.00025 mole) of lithium pyrrolidone was employed as the catalyst. Theisocyanate mixture was dissolved in 20 g. of NMP and the addition timewas 4.5 hours at 80° C. with an overall reaction period of 6 hours atthis temperature. The final copolyimide solution contained about 18percent by weight of solids and was characterized by an ηinh(0.5% at 29°C.)=0.58. Films were easily cast from the NMP solution and whenredissolved in NMP the polyimide had an ηinh(0.5% at 29.2° C.)=0.65.

EXAMPLE 3

The following example is an uncatalyzed polymerization reaction that wasnot carried out in accordance with the present invention but is shownfor purposes of comparison.

Using the procedure and reactants set forth in Example 1 except for thefact that no catalyst was used, the polymerization described therein wasrepeated. At the reaction temperature of 80° C. after 8.75 hours, IRanalysis showed an appreciable quantity of NCO and anhydride groupsremaining. Further, the solution which was 25 percent by weight insolids was quite turbid which was a result of the preferential reactionof the more reactive MDI to form the homo-polyimide which is known to beinsoluble, thereby leaving at least a portion of the TDI unreacted.

EXAMPLE 4

The following example is a polymerization reaction carried out in thepresence of a known catalyst for the reaction of an isocyanate with ananhydride (see J. Drapier, et al., Tetrahedron Letters No. 6, 419-422,1973) but not a catalyst according to the present invention.

Using the procedure and reactants set forth in Example 2, thepolymerization was carried out in the presence of 0.05 g. (0.00015 mole)of dicobalt octacarbonyl. After a 7 hour reaction period at 80° C.,strong bands in the IR absorption spectrum for --NCO and anhydridegroups showed the polymerization was proceeding only at a slow rate. Asin Example 3, the turbidity of the polymerization solution was anindication of the preferential formation of the insoluble MDI basedpolyimide. The dicobalt octacarbonyl did not catalyze thecopolymerization process.

EXAMPLES 5-6

Using the procedure and reactants of Example 1 and substituting thecatalysts set forth in Table I, the copolyimide according to Example 1was obtained in DMF solution in each of the examples.

                  TABLE I                                                         ______________________________________                                                               Polymer Content                                        Catalyst (wt. in g.)   (% by wt.)                                             ______________________________________                                        Ex. 5                                                                              N-potassium valerolactam (0.12)                                                                     25                                                 Ex. 6                                                                              N-potassium caprolactam (0.14)                                                                      25                                                 ______________________________________                                    

EXAMPLE 7

A dry 500 ml. flask equipped as described in Example 1 was charged with38.4 g. (0.2 mole) of sublimed TMA (trimellitic anhydride) and 0.18 g.(0.0015 mole) of potassium pyrrolidone (potassium butyrolactamate)catalyst along with 244 g. of NMP (dried by distillation from calciumhydride). The temperature of the solution was raised to 115° C. whilethe solution was stirred under nitrogen. The addition funnel was chargedwith 51.0 g. (0.204 mole, a 2 mole percent excess) of MDI dissolved in40 g. of NMP and the isocyanate solution was slowly added over 5 hours.A further quantity of MDI, 1.0 g. (a further 2 mole percent excess)dissolved in 4 g. of NMP was added over a 2 hour period. The catalystwas neutralized by the addition of 0.2 g. of glacial acetic acid. Therewas thus obtained a polyamideimide having the recurring unit ##STR9##

Films were cast from the NMP solution and had an average thickness of 3mils.

EXAMPLE 8

A dry 500 ml. resin flask equipped according to Example 1 was chargedwith 49.8 g. (0.3 mole) of purified isophthalic acid and 0.32 g. (0.0026mole) of potassium pyrrolidone. The flask contents were dissolved in 240g. of dry NMP by stirring under nitrogen and the solution heated to 115°C. A solution consisting of 45.0 g. (0.18 mole) of MDI and 20.88 g.(0.12 mole) of a mixture of 80 percent 2,4-TDI and 20 percent 2,6-TDIdissolved in 28 g. of NMP was added to the flask at 115° C. over a 6hour period. An additional 0.84 g. (0.0048 mole) of the 2,4-and 2,6-TDImixture along with 1.75 g. (0.0072 mole) of MDI were diluted with about10 g. of NMP and added to the flask over a 2 hour period. The solutionbecame very viscous and a solution of 0.2 g. of glacial acetic aciddissolved in 92 g. of NMP was added to reduce the solids content toabout 20 percent. There was thus obtained a copolyamide having therecurring unit ##STR10## in which, in 60 percent of the recurring units,R represented ##STR11## and, in the remaining 40 percent, R representeda mixture consisting of 80 percent ##STR12## and 20 percent ##STR13##

EXAMPLE 9

A dry 500 ml. resin flask equipped as in Example 1 was charged with48.82 g. (0.2 mole) of purified brassylic acid, and 0.18 g. (0.0015mole) of potassium pyrrolidone dissolved in 200 g. of dry NMP. Thetemperature was raised to 115° C. and 50.0 g. (0.2 mole) of MDIdissolved in 44 g. of NMP was added dropwise over a 4 hour period. A 1mole percent (0.5 g.) excess of MDI dissolved in 14 g. of NMP was addedover 1 hour. Twelve drops of glacial acetic acid were added to the paleyellow solution to neutralize the catalyst. The polymer solutioncontained 24 percent by weight solids. The polymer was precipitated intowater in a Waring Blendor, collected, washed with acetone, and finallydried by heating at 145° C. in vacuum overnight. There was thus obtaineda polyamide having the recurring unit

I claim:
 1. In a process for preparing an essentially linear, dipolar aprotic solvent soluble solid polyimide by the condensation of an organic diisocyanate with a polycarboxylic compound containing two intramolecular carboxylic anhydride groups or free carboxylic acids thereof under anhydrous conditions in said solvent, the improvement which comprises adding to the solution of polyimide forming reactants a catalytic amount of a compound consisting essentially ofwherein n is an integer from 2 to 5 inclusive, and M is an alkali metal at a temperature of from about 40° C. to about 140° C.
 2. The process according to claim 1 wherein the catalyst is an alkali metal salt of pyrrolidone.
 3. The process according to claim 1 wherein the polycarboxylic acid derivative comprises an aromatic tetracarboxylic acid dianhydride.
 4. In a process for preparing an essentially linear, dipolar aprotic solvent soluble solid polyimide by the condensation of an organic diisocyanate with a polycarboxylic compound containing two intramolecular carboxylic anhydride groups or free carboxylic acids thereof in said solvent, the improvement which comprises adding to the solution of polyimide forming reactants a catalytic amount of a compound consisting essentially of an alkali metal salt of pyrrolidone at a temperature of from about 40° C. to about 140° C.
 5. The process according to claim 4 wherein the polycarboxylic acid derivative is 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride.
 6. The process according to claim 5 wherein the diisocyanate is a mixture comprising from about 10 to about 35 mole percent 4,4'-methylenebis(phenylisocyanate) and about 65 to about 90 mole percent of a member selected from the group consisting of 2,4-toluenediisocyanate, 2,6-toluenediisocyanate, and mixtures thereof.
 7. The process according to claim 6 wherein the diisocyanate is a mixture comprising about 20 mole percent 4,4'-methylenebis(phenylisocyanate) and about 80 mole percent of a member selected from the group consisting of 2,4-toluenediisocyanate, 2,6-toluenediisocyanate, and mixtures thereof.
 8. The process according to claim 7 wherein the dipolar aprotic solvent is N-methylpyrrolidone.
 9. The process according to claim 7 wherein the dipolar aprotic solvent is dimethylformamide.
 10. The process according to claim 7 wherein the catalyst is lithium pyrrolidone.
 11. A process according to claim 1 wherein said polyimide remains dissolved in said dipolar aprotic solvent. 